Fossil-fuel power plants produce the main part of the energy actually consumed worldwide. Energy is generated from the combustion of fossil-fuels such as coal, natural gas and fuel oil. The use of biomass to fuel the power plant is also within the scope of this invention. Main exhaust gases formed from such processes may be CO2, SO2 and NOx depending on the nature of the fuel used. Treatment systems are already available for reducing SO2 and NOx emissions. However to date, CO2 emissions from fossil-fuel power plants are generally not contained or reduced. These CO2 emissions thus contribute to increase the atmospheric concentration of CO2, the most important greenhouse gas. It is known that such an increase in greenhouse gases causes climate changes which could lead to various environmental problems, such as an increase in violent weather events, significant temperature warming in specific areas, changes in the precipitation pattern trends and a rise of ocean level.
Moreover, in the next century, a significant increase of carbon dioxide concentrations is expected, unless energy production systems reduce their emissions into the atmosphere. Carbon sequestration consisting of carbon capture, separation and storage or reuse represents potent ways to stabilize and eventually reduce concentration of atmospheric CO2.
Several technologies, based on carbon sequestration, are being studied by academic and industrial groups. These are: transformation by algae, sequestration in oceans, storage in depleted oil and natural gas wells and dissolution of pressurized CO2 in water tables. CO2 can also be transformed into more geologically stable forms, such as calcium carbonate.
Transformation of CO2 with algae involves the use of algal photosynthesis. The gas emitted by power stations is thus directly introduced in basins located nearby. The selected algae must therefore support these environments with harsh conditions. The algae produced could be dried up and used as fuel to supply the power station. This approach reduces the required fuel to supply power, but does not eliminate CO2 production completely.
Sequestration in oceans consists in pumping the carbon dioxide to be disposed of to depths of 1,000 metres below sea level. The technique is based on the fact that CO2 possesses a higher density than water. It is believed that CO2 will sink to the bottom of oceans where lakes of liquid carbon dioxide will be formed. However, as yet, the environmental impact of this technology has not been demonstrated (U.S. Pat. No. 6,475,460). Another way is to bring carbon dioxide and seawater or fresh water into contact to form carbon dioxide hydrate and sinking it in the seawater, fresh water or geological formation under conditions for the stability of carbon dioxide hydrate (CA 2,030,391, patent application US 2003/0055117, patent application US 2003/0017088; U.S. Pat. No. 6,254,667).
Oil and natural gas wells are capable of supporting enormous pressures without leakage. They are therefore an ideal location for the storage of compressed CO2 (patent application CA 2,320,216; U.S. Pat. No. 6,598,398; U.S. Pat. No. 6,389,814; U.S. Pat. No. 6,170,264). In the petroleum industry, the injection of CO2 in wells to enhance oil recovery is a widely used technique. However, this method only constitutes a temporary storage, since in the medium term, the displacements of the earth crust are capable of bringing about a release of CO2. Moreover, although there are hundreds of depleted sites around the world, their total capacity is after all limited, and there is an obligation to land case the geological formations involved.
The deep water tables are distributed throughout the globe. They generally include salt water and are separated from the surface water tables which constitute the drinking water supplies. The water contained in these natural reservoirs can dissolve the pressurized CO2 and even disperse it in the geological formations. However, the implementation of this technology must always imply a strong concern regarding the proximity of the water tables with the CO2 emission sources.
CO2 sequestration in solid carbonates and/or bicarbonates has already been reported in Lee et al. (U.S. Pat. No. 6,447,437). However, CO2 chemical transformation into bicarbonate fertilizer requires methane, hydrogen and nitrogen. Kato et al. (U.S. Pat. No. 6,270,731) reported CO2 sequestration into carbon powder. However, methane and hydrogen are required. Shibata et al. (U.S. Pat. No. 5,455,013) reported CO2 sequestration into CaCO3. However, the chemical process enables CO2 sequestration into CaCO3 only. Other carbonates cannot be obtained.
Although some solutions have been proposed in the past for reducing CO2 emissions in general, few of them have shown to be efficient or commercially viable for different reasons. Moreover, a very few, if not none, of the solutions proposed specifically apply to CO2 emissions from fossil-fuel power plants. Thus, there is still a need for a solution for reducing those CO2 emissions from fossil-fuel power plants. With the general concern throughout the world with respect to the urgency of finding a solution to the problem of emissions of greenhouse gases, this need is even more obvious.